1. Field of the Invention
The present invention relates generally to communication between multiple devices over a single, shared transmission medium, and more specifically to employing an adaptive medium access control protocol together with a corresponding logical link control protocol supporting adaptive channel allocation to improve device communication over a shared transmission medium.
2. Description of the Prior Art
Several access control methodologies exist for transmission on multi-access channels within a single transmission medium. Token ring and token bus schemes control access to the transmission medium in an orderly fashion designed to maximize bandwidth utilization and to maintain fairness (priorities) and determinism. Token ring and token bus schemes may be viewed as forms of multiplexing (MUXing), which may be loosely defmed as combining two or more information channels onto a common transmission medium. Based on the premise that a transmission medium""s speed and capacity far exceed a single user""s requirements on any end of the communication medium, and the logical conclusion that several transmitting entities may be able to utilize the same transmission medium, multiplexing typically divides the medium transmission time into xe2x80x9ctimeslotsxe2x80x9d. Each timeslot is then uniquely assigned to a single transmitting entity, which owns the medium for the full duration of the timeslot and may transmit on the medium only during that assigned portion of time, and is the only transmitting entity permitted to transmit during that assigned portion of time. Time division multiplexing (TDM) thus achieves shared access to a single transmission medium by defmed division of transmission time among the transmitting entities. In environments where usage requirements of transmitting entities may vary significantly at run time, statistical time division multiplexing (STDM), in which timeslot duration is not predetermined or fixed but instead varies at run time, may alternatively be employed.
Ethernet-type medium access control schemes, on the other hand, generally allow a network element to transmit at will on the transmission medium. While non-deterministic in nature, this system possesses several attractive characteristics, including simplicity, support for dynamic changes in network element population, autonomous network element operation, low transmission latency at low utilization levels, and acceptable throughput under average loading conditions. Drawbacks of Ethernet include degradation of throughput under heavy traffic loading, non-determinism, and absence of priority assurance. Ethernet throughput rates are typically in the range of 20% to 50% depending on the specific implementation, and drop drastically from those levels when the transmission medium is heavily loaded. Current Ethernet transmission medium access control utilizes the truncated binary exponential back-off algorithm, a simple algorithm by which a transmission controller chip may adapt access to the medium according to the medium loading condition. The basic outcome of the algorithm is minimal latency and acceptable throughput and utilization under light loading conditions, with increased latency and acceptable throughput and bandwidth utilization as traffic increases towards heavy traffic conditions.
It would be desirable, therefore, to provide a shared medium control access methodology which exploits the benefits of Ethernet-type systems while providing some levels of determinism, priority, and sustained throughput efficiency with increasing traffic loads. It would also be advantageous for the methodology to conduct adaptive channel allocation of transmission jobs into virtual channels on the medium.
A hybrid transmission cycle (HTC) unit of bandwidth on a shared transmission medium is defined to include an adaptive, time division multiplexing transmission cycle (ATTC), which is allocated in portions sequentially among all participating network entities, and a residual transmission cycle (RTC), which is allocated in portions, as available, to the first network entity requesting access to the shared medium during each particular portion. The ratio of logical link virtual channels, or D-Channels, to data payload virtual channels, or B-Channels, within the ATTC is adaptive depending on loading conditions. Based on transmission profiles transmitted on the D-Channels during the ATTC, each network entity determines how many B-Channels it will utilize within the current HTC. This calculation may be based on any decision network, such as a decision network modelling the transmission medium as a marketplace and employing microeconomic principles to determine utilization. The ratio of the duration of the ATTC segment to the duration of the RTC segment is also adaptive depending on loading conditions, to prevent unacceptable latency for legacy network entities employing the shared transmission medium. During the RTC, utilization of the shared medium preferably reverts to IEEE 802.3 compliant CSMA/CD transmission, including transmissions by HTC-compliant network entities.